Epigenetic inheritance of transcriptional programs is essential for cell proliferation, cell differentiation and malignant transformation. Molecularly, epigenetic inheritance of transcriptional programs relies in large part on conservation of chromatin structure through disruptive cell cycle phases. Despite the biological importance of this phenomenon, the details of the molecular mechanisms of epigenetic inheritance are elusive, mostly because of the lack of appropriate experimental approaches. We developed new experimental paradigms, and found that many chromosomal proteins are associated with DNA during replication. However, methylated histones are accumulated on nascent DNA with significant delay. Our preliminary results suggest that following DNA replication, undifferentiated cells, including mouse embryonic stem cells (mESCs), accumulate methylated histones and resume transcription more slowly than differentiated cells of the same origin. We hypothesize that this may create a narrow time window at the time of DNA replication, with a uniquely open chromatin configuration lacking histone modifications and lacking transcription that is very susceptible to receiving signals for differentiation. We will test this hypothesis by examining th accumulation of different modified histone residues, histone-modifying and chromatin remodeling proteins following DNA replication during differentiation of mouse ESCs. We will also examine when transcription resumes in these cells, and when components of the transcriptional machinery are recruited to nascent DNA. Finally, we will test a hypothesis that the short period of time just after DNA replication, when chromatin is not modified, provides a window of opportunity for acquiring signals for differentiation through association of the newly induced transcription factors. This may account for a higher plasticity of undifferentiated states in acquiring new transcriptional programs. Overall, this proposal will greatly contribute to the ke epigenetic issues during cell differentiation.